The present invention generally relates to fabrication of semiconductor devices and more particularly to an electron-beam lithography used for fabricating ultra-fine semiconductor devices. More particularly, the present invention relates to the creation and representation of exposure data used in an electron-beam exposure system when fabricating ultra-fine semiconductor devices. Further, the present invention relates to the fabrication process of an exposure mask.
An optical lithography process, which exposes numerous semiconductor patterns simultaneously while using a single mask, is an efficient exposure process of semiconductor devices and has been used extensively in production of various semiconductor devices and integrated circuits.
However, increasing demand of device miniaturization has led to such a situation in which a semiconductor pattern has to be exposed with a size comparable with a wavelength of the optical radiation that is used for the exposure. In order to deal with such a stringent demand of device miniaturization, various super-resolution techniques, including the art of phase shift mask, modified optical illumination, or use of deep ultraviolet wavelength radiation, has been proposed. However, such a measure of super-resolution technique is now reaching its limit and the difficulty of exposing a large area by a single exposure shot is increasing sharply. Associated therewith, the throughput of exposure is decreasing substantially. In the case of exposing 256 Mbit DRAMs, for example, the actual throughput of exposure is estimated to be about 30 wafers per hour.
The art of electron-beam lithography is a process that is expected to overcome the foregoing problem of resolution and throughput. In an electron-beam exposure process, in which the exposure is conducted by a finely focused electron beam, it is possible to expose extremely fine patterns smaller than 0.1 .mu.m on a semiconductor wafer. Thus, an electron-beam exposure process has been used for producing a limited amount of prototype semiconductor devices of various, different designs. For such a purpose, a direct electron-beam exposure process, which does not use an exposure mask, is advantageous.
Thus, the electron-beam exposure process is currently used in various fields of semiconductor industry, ranging from LSI development to preparation of exposure mask. On the other hand, due to the nature of the direct electron-beam exposure process of exposing a pattern consecutively by a single focused electron-beam, the use of such a direct electron-beam exposure process is not appropriate for mass producing semiconductor devices. The throughput of exposure is deteriorated further when the cross-sectional shape of the electron-beam is changed by a variable-beam shaping process, in which the electron-beam is displaced with respect to an aperture mask for the variable-beam shaping.
FIGS. 1A and 1B show an example of such a variable-beam shaping of an electron-beam.
Referring to FIG. 1A, an electron beam BM1, shaped to have a large rectangular cross-section by a first mask M1, is passed through a second mask M2 to form an electron beam BM2 of a smaller, rectangular cross-section. The size of the electron beam BM2 is changed by changing the optical axis of the electron beam BM1 with respect to the mask M2. Thus, by using the electron beam BM2, it is possible to expose various patterns including a triangular pattern as indicated in FIG. 1B, wherein it can be seen that the oblique edge of the triangular pattern is represented by a number of steps. It should be noted that the exposure of the triangular pattern is conducted by carrying out a number of shots each exposing a small rectangle by the electron-beam BM2. Thus, such a variable-beam shaping process has a drawback in that it takes a long time for exposing a desired pattern. For example, only one or two wafers are exposed per hour.
Meanwhile, there is a proposal of so-called block exposure process in which various patterns are exposed on a wafer by shaping the electron beam according to the desired pattern.
FIG. 2 shows the principle of the block exposure process.
Referring to FIG. 2, it can be seen that the second mask M2 called hereinafter a block mask carries a number of apertures P or Pa of various shapes for shaping the electron-beam BM1. As the electron-beam BM2 thus shaped by the block mask M2 has a cross-sectional shape corresponding to the shape of the aperture hit by the electron-beam BM1, it is possible to expose the wafer with the shape of the selected aperture by directing the electron-beam BM1 such that the electron-beam BM1 hits the desired aperture P and by focusing the shaped electron-beam BM2 on the semiconductor wafer with a demagnification. For example, it is possible to expose a triangular pattern or cross-shaped pattern on the semiconductor wafer by a single shot. Thus, the block exposure process of FIG. 2 is suited for mass production of semiconductor devices. In the case of producing 256 Mbit DRAMS, for example, a throughput of 20 wafers or more per hour can be achieved.
In the block exposure process of FIG. 2, it is still required to provide a beam shaping mask including the masks M1 and M2. In the block mask M2, in particular, there arises a problem in that, while the exposure of stripe patterns shown in FIG. 3B is made successfully by using the block mask of FIG. 3A, on which a number of stripe patterns are formed, the exposure of a ring-shaped pattern including an isolated dot such as the one shown in FIG. 3D is not possible, as the corresponding block mask shown in FIG. 3C cannot be formed in practice. It should be noted that the central dot of the block mask of FIG. 3C corresponding to the isolated dot of FIG. 3D lacks a mechanical support and cannot be maintained. As a result, the pattern actually exposed on the semiconductor wafer becomes a mere flat exposure pattern as indicated in FIG. 3E.
In addition to the foregoing difficulty of preparing a block mask, the block exposure process of FIG. 2 has a further difficulty, associated with the fact that the shaped electron-beam BM2 is demagnified by a factor of tens or hundreds, in that the electrons in the shaped electron beam BM2 tend to repel each other due to Coulomb repulsion occurring in the electrons as a result of the severe focusing of the electron-beam for the demagnification. When such a Coulomb repulsion takes place in the electrons in the electron beam BM2, the resolution of the exposed pattern is deteriorated inevitably.
In order that the block exposure process provides the expected high-throughput exposure, it is necessary that the block mask M2 carries frequently used exposure patterns. As the number of the exposure patterns that can be formed on the block mask M2 is limited in view of the limited size of the block mask M2, it is essential to make sure that the exposure patterns on the block mask are the patterns used most frequently. Otherwise, the block mask M2 has to be replaced frequently, while such a replacement of the block mask deteriorates the throughput of exposure substantially. However, such extraction of the frequently used patterns from a huge number of possible exposure patterns is difficult for a human operator.
In the actual block exposure process conducted by an electron-beam exposure apparatus, it is practiced to prepare an evaluation pattern on the block mask M2 for evaluation of the exposure pattern exposed on the semiconductor wafer by the evaluation pattern. The evaluation pattern is thereby used for various adjustment of the electron-beam exposure apparatus, including smooth interconnection of a block exposure pattern and a variable-beam exposure pattern, compensation of exposed pattern variation caused by the block mask or by the semiconductor wafer, and the like. However, such adjustment of the electron-beam exposure apparatus has been difficult to carry out manually, as such a manual adjustment of the electron beam exposure apparatus takes an enormous time, which inevitably leads to a delay in the setup of the exposure process.
In addition, conventional block exposure process has a problem of proximity effect. When a proximity effect is caused, the electrons focused upon the semiconductor wafer are back-scattered by the resist covering the semiconductor wafer or the semiconductor wafer itself, and the exposed pattern is distorted. In order to compensate for such a distortion, it is necessary to change the exposure dose depending on the exposed pattern. In the case of block exposure process, on the other hand, such a mere change of the exposure dose depending on the exposed pattern is not sufficient, as the degree of back-scattering of the electrons is influenced not only by the irradiation dose but also by the adjacent exposure patterns.
Further, it should be noted that the amount of the exposure data to be processed when exposing an LSI pattern on a semiconductor wafer by using an electron-beam exposure apparatus, is enormous. Thus, several days have been necessary to process the exposure data before conducting the actual exposure process, while such a long process time of the exposure data decreases the throughput of production of the semiconductor devices.
It is not possible to say what pattern on the semiconductor wafer has been exposed by the block exposure process and what pattern on the same semiconductor wafer has been exposed by the variable-beam exposure process. Thus, it has been necessary to test the block mask by first forming the block mask and then conducting an exposure process while using the block mask thus formed. However, such an evaluation process is expensive and takes time.